The present invention relates to an oil injection type scroll fluid machine suited for a refrigeration compressor for air conditioning, a cryogenic helium refrigerator or an air compressor.
Generally, a scroll fluid machine includes an orbiting scroll member having an end plate and a vertical wrap extending from one surface of the end plate and formed along an involute curve or a curve which approximates to an involute curve, and a stationary scroll member, of a similar construction provided with a discharge port and a suction port formed in a central and an outer peripheral portion of the end plate. The orbiting scroll member and the stationary scroll member are brought together in such a manner that the wraps thereof mesh with each other.
An Oldham's mechanism is disposed between the orbiting scroll member and the stationary scroll member or a frame carrying the stationary scroll member so as to prevent the orbiting scroll member from rotating about its own axis. An eccentric pin on a rotary shaft engages with the orbiting scroll member through a bearing.
In operation, the eccentric pin revolves as the rotary shaft is driven so that the orbiting scroll member makes an orbital movement without rotating about its own axis, whereby a gas is drawn into and compressed in enclosed compression spaces formed by the walls of the wraps and the end plates of both scroll members. This type of scroll fluid machine is shown, for example, in the specification of U.S. Pat. No. 3,884,599.
During the operation of the scroll fluid machine of the aforementioned type gas under compression in the enclosed spaces between both scroll members produces a force which acts to separate both scroll members in the axial direction and the tendency of both scroll members to move away from each other will cause a leak or by-passing of the gas under compression to a lower pressure space thereby adversely affecting the compression performance.
In order to avoid the above-noted problem, in, for example, Japanese Laid Open Patent Application No. 55-148994, it has been proposed to apply a gas pressure to the back side of the end plate of the orbiting scroll member so as to axially press the orbiting scroll member onto the stationary scroll member. For this purpose a part of the gas of an intermediate pressure between the suction pressure and the discharge pressure is extracted or withdrawn and introduced to the back side of the orbiting scroll member so as to obtain the axial pressing force, thereby attaining a high sealing effect between both scroll members.
In, for example, Japanese Laid Open Patent Application No. 56-85087, scroll fluid machine is proposed wherein an oil is injected into an enclosed space or spaces formed between both scroll members, in order to cool the gas under compression during the operation of the machine.
A disadvantage of the above-noted scroll fluid machine resides in the fact that when the scroll fluid machine is being started up or is going to be stopped, the oil undesirably fills the enclosed space or spaces adapted to be injected thereby resulting in a risk of breakdown of the scroll wraps due to liquid compression, or a forcible moving of the orbiting scroll member from the stationary scroll member due to an abnormal rise of the internal pressure in the enclosed space or spaces, resulting in a compression and starting failure.